Currently, mobile communication networks are primarily CS networks, for example, Global System for Mobile communications (GSM) and Code Division Multiple Access (CDMA) networks. Operators have set up perfect and diverse service platforms based on CS networks. On such platforms, a Mobile Switching Center (MSC) is responsible for routing calls and executing service logics.
An IP Multimedia Subsystem (IMS) is a service network based on Internet Protocol (IP) switching. The IMS allows a User Equipment (UE) to access an IMS network through various PS access networks (such as an IP Connectivity Access Network (IP-CAN)) and carry out IMS multimedia services. That is, the IMS is a service platform built on the IP-CAN, and basically corresponds to the MSC of the CS network. Compared with the CS network, the IP-CAN provides higher bandwidth and supports richer services. The core of the IMS is the Call Session Control Function (CSCF) and various Application Servers (ASs). A Serving-CSCF (S-CSCF) is responsible for routing a call request to an appropriate AS when conditions are fulfilled, and the AS executes service logics.
Because the IMS is a megatrend, it is appropriate that the functions of the CS network service platform are transferred to the IMS network to unify the service platform and reduce the costs of developing and operating new services. The unification of service platforms is also known as an IMS Centralized Service (ICS).
Service Continuity (SC) deals with session continuity that appears when a user moves from one access network to another. That is, the access network can be changed without interrupting the session when the user moves.
An SC UE performs session transfer between access networks according to the following procedure: At the new access network, the UE sends a transfer request to a Service Centralization and Continuity Application Server (SCC AS), with a view to setting up a new access leg between the UE and the SCC AS in the new access network. After receiving the transfer request, the SCC AS associates a remote leg corresponding to the access leg of the old access network with the access leg of the new access network, and updates the remote leg and releases the access leg of the old access network at the same time. The remote leg refers to a call control path between the SCC AS and the peer UE. The access leg and the remote leg are defined in 3GPP TS23.237.
A Session Transfer Identifier (STI) is an identifier indicating initiation of session transfer. Generally, when a new access leg is set up between the UE and the SCC AS, the SCC AS allocates the STI dynamically. Each dynamic STI corresponds to a specific access leg uniquely. When the user wants to perform handover, if a dynamic STI exists in the UE, the UE sends a transfer request by using the dynamic STI as a called number. The SCC AS determines the old access leg which needs to be replaced by the new access leg according to the STI.
A static STI and a static Session Transfer Number (STN) are stored in each SC UE. When the UE has no dynamic STI, the static STI or STN is used as a called number to initiate transfer. The SC UE may be an ICS UE (UE capable of ICS) or non ICS UE (UE incapable of ICS). If the SC UE is a non ICS UE, when the UE sends a session request from the CS network, the dynamic STI allocated by the SCC AS to the new access leg cannot be transmitted to the UE. Therefore, only the static STI can be used when the UE initiates transfer from the CS network to the PS network. The STN can be used only if the SC UE is a non ICS UE and the transfer is initiated from the PS network to the CS network.
The STN, the static STI, and the dynamic STI are different in structure. The SCC AS can easily identify whether the user uses an STN, or a static STI, or a dynamic STI.
In the process of implementing the present invention, the inventor finds at least the following problems in the prior art:
When a non ICS UE moves from a CS network to a PS network, because no dynamic STI exists, only the static STI is available, and only one static STI is stored in the UE. When the non ICS UE has two multimedia sessions in the CS network, the non ICS UE can transfer only the CS session which is currently active, but is incapable of transferring the held session, for no more STI exists.